Interconnect members for integrated circuits are known which include aluminum or aluminum based alloys layered with transition metals such as titanium. J. Howard et al., "Intermetallic Compounds of Al and Transition Metals: Effect of Electromigration in 1-2 .mu.m Wide Lines," J. Appl. Phys., Vol. 49, p. 4083 (1978); U.S. Pat. No. 4,017,890 issued Apr. 12, 1977, to Howard et al., entitled, "Intermetallic Compound Layer in Thin Films for Improved Electromigration Resistance;" U.S. Pat. No. 4,154,874 issued May 15, 1979, to Howard et al., entitled, "Method for Forming Intermetallic Layers in Thin Films for Improved Electromigration Resistance;" and S. Iyer et al., "Electromigration Study of the Al-Cu/Ti/Al-Cu System," 22nd Ann. Int'l Reliability Phys. Symp., Conf. Proc. (Las Vegas, 1984).
Interconnects formed of aluminum and aluminum alloys, however, suffer from certain difficulties which have been associated with mass transport or diffusion of aluminum atoms. One such problem is the formation of protrusions, commonly known as hillocks, which form during high temperature processing steps, typically in the range of 200.degree. C. to 450.degree. C. These hillocks are formed to relieve the compressive stresses generated in the aluminum by the differential thermal expansion between the aluminum and the silicon substrate wafer. Another problem is that of electromigration failures caused by transport of atoms under the influence of an electric current. In addition, mechanical stress voids created by creep or the transport of atoms under the influence of high tensile stress also lead to interconnect failures.
Layering an interconnect member with titanium has been found to completely eliminate vertical hillocks during thermal processing steps. D. Gardner et al., "Layered and Homogeneous films of Aluminum and Aluminum/Silicon with Titanium and Tungsten for Multilevel Interconnects,"IEEE Trans. Elect. Devices, Vol. ED-32, p. 174 (1985). Layering with titanium has also reduced failures caused by electromigration. However, it has been found that titanium layered interconnect members often suffer passivation dielectric cracking prior to an electrical open circuit failure due to the mass transport of aluminum atoms within each separate layer of the interconnect. More specifically, aluminum protrusions from some of these cracks have been observed. This is described by R.E. Jones, Jr. and L.D. Smith, "Contact Spiking and Electromigration Passivation Cracking Observed for Titanium Layered Aluminum Metallization," IEEE VLSI Multilevel Interconnect Conf., Conf. Proc., p. 192 (Santa Clara 1985). Therefore, passivation cracking can result in two reliability problems; that of electrical shorting between adjacent interconnect members resulting from the aluminum protrusions, and corrosion of the exposed metal interconnect members through the broken passivation layer. These lateral protrusions are caused by mass transport of aluminum atoms within each aluminum-silicon layer under the influence of an electric current. Various sources of nonuniformity result in mass depletion in some regions and mass accumulation in other regions. The mass accumulation can results in lateral hillocks, especially since the ability to form vertical protrusions is restricted by the titanium layer or layers. Eliminating or reducing these lateral aluminum protrusions would prevent both problems of passivation layer cracking and electrical shorting between adjacent interconnect members.